The invention relates to a process for native potato protein isolation, to native potato protein isolates, to the use thereof, and to a food, nutraceutical and pharmaceutical product comprising a native potato protein isolate.
The undiluted juice from potato tuber is called potato fruit juice. Potato fruit juice may be produced by washing and rasping potatoes and separating the starch and fibres by various techniques, such as centrisieves, hydrocyclones and decanters. Fresh potato fruit juice is a complex mixture of soluble and insoluble material comprising proteins, starch, minerals, toxic glycoalkaloids and monomeric and polymeric reactive phenols.
Fresh potato fruit juice is however not very stable. Oxidation leads to conversion of phenolic compounds into quinones which rapidly combine into a dark polymer residue. During the oxidation process reaction the potato proteins can partially cross-link, which dramatically reduces the solubility of the proteins. The complexity and instability of the potato fruit juice makes the separation and isolation of minimally denatured or modified proteins a complicated and economically demanding process.
Native potato proteins can tentatively be divided into three classes (i) the patatin family, highly homologous acidic 43 kDa glycoproteins (40-50 wt. % of the potato proteins), (ii) basic 5-25 kDa protease inhibitors (30-40 wt. % of the potato proteins) and (iii) other proteins mostly high molecular weight proteins (10-20 wt. % of the potato proteins) (Pots et al., J. Sci. Food. Agric. 1999, 79, 1557-1564). Potato protein is rich in lysine and may form an excellent supplement for lysine-poor proteins such as those of cereal. The nutritional value of total potato protein have been shown to be greater than that of casein and comparable to that of whole egg white protein.
Patatin is a family of glycoproteins that have lipid acyl hydrolase and transferase activities and accounts for up to 40 wt. % of the total soluble protein in potato tubers.
Protease inhibitors can be divided into different groups based on their molecular weight. The different groups of protease inhibitors are identified as protease inhibitor I (molecular weight of about 39 kDa), carboxypeptidase inhibitor (molecular weight of about 4 100 Da), protease inhibitors IIa and IIb (molecular weight of about 20.7 kDa), and protease inhibitor A5 (molecular weight of about 26 kDa). The ratio of these different groups of protease inhibitors in the total potato protein depends on the potato variety. Protease inhibitors from potato have a broad range of potentially important applications. Protease inhibitors have for instance shown to be useful in the treatment of diabetes, for eliciting satiety in mammals, for reducing the risk of skin cancer, for inhibiting the growth of bacteria, and for preventing or treating inflammation on pruritis of skin and intestine, see for instance WO-A-99/059623.
Despite its unique nutritional qualities, potato protein is currently only used as animal feed, because the available products exhibit a number of serious drawbacks.
One of the major drawbacks is that the recovery of potato protein from the effluent of potato starch mills is typically carried out on an industrial scale by heat coagulation. During the heat coagulation process, the potato proteins become heavily denatured and as a consequence lose functional properties that are required for applications in the food, nutraceutical and pharmaceutical industry, such as solubility in water.
Other, milder methods for recovering potato proteins, such as membrane filtration applied directly to potato fruit juice and precipitation methods show a low purity and a lack of selectivity and are unable to separate functionalities, see for instance WO-A-97/42834.
There is a commercial interest in a process for producing native total potato protein isolate, native patatin isolate and native protease inhibitor isolate. The term “native potato protein” is used in this application is meant to refer to the potato protein without any significant physical or (bio)chemical modification or inactivation, in particular denaturation. Existing methods for isolating potato proteins and potato protein fractions include fractionation, ion exchange, gel permeation, ultrafiltration, affinity and mixed-mode chromatography and fractionation by heat coagulation. A disadvantage of these prior art isolation methods is that they lack a strict pH control to maintain good potato protein characteristics. Furthermore, they do not sufficiently deal with undesirable contaminants. In particular, glycoalkaloid contaminants are not sufficiently removed.
Glycoalkaloids are well-known anti-nutritional factors. The glycosylated forms (such as α-solanine and α-chaconine) show the highest toxicity. The aglycons (such as solanidine), have a more than 100 fold lower liver toxicity. α-Solanine, α-chaconine and derivatives constitute for more than 95% of the glycoalkaloids in the potato. Other glycoalkaloids include for example tomatine, tomatidenol and demissidine.
Glycoalkaloids have a bitter taste and negatively affect many of the physical and/or biological properties of the proteins, especially when the pH is increased by adhering to the soluble proteins as shown in this application. For food applications the taste threshold is about 140-170 mg of glycoalkaloids expressed as α-solanine per kg product. This threshold strongly limits the applications of prior art native potato protein isolates in foods.
The present inventors have found that the poor solubility of glycoalkaloids such as α-solanine at pH values typically above 6.2 results in an excessive adherence of glycoalkaloids and other compounds, such as polyphenols, to the potato proteins.
Partial removal of glycoalkaloids has been achieved by various ultrafiltration methods at excessive diafiltration conditions, see for instance WO-A-97/42834. The HPLC method employed (Friedman M. et al., J. Agric. Food Chem. 2003, 51, 2964-2973 or Houben et al., J. Chromatogr. A 1994, 661, 169-174) does not detect the aglycons that are formed by enzymatic hydrolysis after prolonged processing of potato fruit juice as described. Ultrafiltration can remove some glycoalkaloids and salts, but does not remove high molecular contaminants, such as polyphenols and proanthocyanidines and coloured derivatives thereof, such as epicatechins and anthocyanines, that are formed at pH values below 4.5.
Glycoalkaloids can also be removed by enzymatic hydrolysis. However, this does not lead to removal of aglycon, which also binds to the potato proteins with negative effects on their physical and/or biological properties. Therefore, both a HPLC method (Friedman M. et al., J. Agric. Food Chem. 2003, 51, 2964-2973) and a colorimetric method (Walls et al., J. Chem. Ecol. 2005, 31, 2263-2288) must be used to measure the main glycoalkaloids, the total glycoalkaloids and the aglycons, respectively.
Glycoalkaloid removal by fermentation is not considered relevant for safe native protein production. Conversion by fermentation causes severe technical issues to implement this process at a commercial scale. The bioconversions are costly and have a low productivity. The micro-organisms employed and hygiene are limitations for the application of the derived products for foods.
Other undesirable contaminants in the native potato protein isolate are for instance pectins, proanthocyanidines and fatty acids. Pectins typically lead to flocculation of the isolate at pH values below 5.0.
Sufficiently pure native potato protein fractions of patatin and protease inhibitor cannot be obtained by partial heat coagulation.
Contaminants, such as polyphenols, in the patatin fraction lead to very variable physical properties in terms of colour and solubility at various pH. The presence of protease inhibitor and other contaminants with a surface active function in the patatin fraction has a negative effect on the good emulsifying and gelling properties of patatin. Also, further purification of the patatin fraction improves the foaming and emulsification properties, as well as the foaming and emulsion stability of patatin.
On the other hand, a protease inhibitor fraction containing patatin contaminants is less useful in e.g. pharmaceutical applications.
Accordingly, there remains a need for an efficient process for isolating native potato protein and native potato protein fractions that have a high degree of purity.